#include <bits/stdc++.h>

#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define in read<int>()
#define lin read<ll>()
#define rep(i, x, y) for(int i = (x); i <= (y); i++)
#define per(i, x, y) for(int i = (x); i >= (y); i--)

using namespace std;

using ll = long long;
using db = double;
using pii = pair < int, int >;
using vec = vector < int >;
using veg = vector < pii >;

template < typename T > T read() {
	T x = 0; bool f = 0; char ch = getchar();
	while(!isdigit(ch)) f |= ch == '-', ch = getchar();
	while(isdigit(ch)) x = x * 10 + (ch ^ 48), ch = getchar();
	return f ? -x : x;
}

template < typename T > void chkmax(T &x, const T &y) { x = x > y ? x : y; }
template < typename T > void chkmin(T &x, const T &y) { x = x < y ? x : y; }

const int N = 2e5 + 10;

int a[N], b[N], n;
ll pre[N], suf[N];

int main() {
#ifndef ONLINE_JUDGE
	freopen("1.in", "r", stdin);
#endif
	n = in; rep(i, 1, n) a[i] = in;
	n--; rep(i, 1, n) b[i] = a[i + 1] - a[i];
	rep(i, 1, n) pre[i] = pre[i - 1] + (b[i] <= 0 ? -b[i] + 1 : 0);
	per(i, n, 1) suf[i] = suf[i + 1] + (b[i] >= 0 ? b[i] + 1 : 0);
	ll ans = 1e18;
	rep(i, 0, n) chkmin(ans, -min(pre[i], suf[i + 1]) + pre[i] + suf[i + 1]);
	printf("%lld\n", ans);
	return 0;
}
